The field of the present invention is front suspension forks for two-wheeled vehicles and more particularly the telescopic mounting of the struts thereof.
Telescopic front fork systems have been used widely as front suspensions for two-wheeled vehicles such as motorcycles. Such a front suspension is illustrated in FIG. 1 where a first strut 10 and a second strut 12 are arranged in parallel to support a wheel 14 on an axle 16. The struts 10 and 12 each include an inner tube 18 and an outer tube 20. The inner tube 18 and the outer tube 20 are partially telescoped together. The inner tubes 18 of each strut are held rigidly in parallel by an upper bridge 22 and a lower bridge 24. A steering stem 26 is also held in place by the upper and lower bridges 22 and 24 to form a pivot about which the front fork may be pivoted for steering.
A conventional strut for a telescopic front fork is illustrated in detail in FIG. 2. This conventional strut, generally designated 28, includes an outer tube 20 and an inner tube 18 partially telescoped together. A stud pipe 30 is fixed to the outer tube 20 by means of a fastener 32. The stud pipe 30 extends within the inner tube 18 and includes an enlarged head or piston 34 at one end. The upper end of the inner tube 18 is closed by a bolt 36. A suspension spring 38 is enclosed within the inner tube 18, extending between the bolt 36 and the piston 34. Thus, as the outer tube 20 rises relative to the inner tube 18, the suspension spring 38 is compressed between the bolt 36 and the piston 34 to provide a resilient cushioning of the vehicle.
To mount the inner tube 18 within the outer tube 20, two metal bushings 40 and 42 are placed between the tubes 18 and 20. The bushings 40 and 42 are spaced apart to provide additional strength against bending forces. The upper metal bushing 40 is positioned within a groove in the inner surface of the outer tube 20 defined by a shoulder 44 on one side and a seal 46 and spring retainer 48 on the other. The lower bushing 42 is positioned within an annular groove formed in the inner tube 18 as illustrated.
A damping function is accomplished employing a fluid 50 which is allowed to flow through a portion of the stud pipe 30 in a central passageway 52 and through holes 54 to the cavity defined between the stud pipe 30 and the inner tube 18. A valve assembly 56 installed at the lower end of the inner tube 18 dampens the motion of the strut by transforming the compression and extension energy produced into thermal energy.
The foregoing conventional strut for a front fork system requires relative sliding between the inner tube 18 and the outer tube 20 as borne by the metal bushings 40 and 42. Because of the variations in frictional forces between the bushings 40 and 42 and the inner and outer tubes 18 and 20, a consistant and smooth extension or contraction of the strut is not necessarily experienced. This movement may not be sufficiently smooth because of the difference between static friction of the metal bushings 40 and 42 against the tubes 18 and 20. Once sliding friction commences, relativj ly smooth motion is experienced.
In order to improve the ride, it would be advant geous to overcome the static friction to reduce that friction to a level comparahle to the sliding friction of the components. However, such frictional characteristics depend to a large extent on the materials employed, the lubricant employed and the retention of lubricant between the sliding surfaces. Such factors are often difficult to control.